sulfonyl chloride

Copper(II) chloride

Structure

Anhydrous CuCl2 adopts a distorted cadmium iodide structure. Most copper(II) compounds exhibit distortions from idealized octahedral geometry due to the Jahn-Teller effect, which in this case describes the localisation of one d-electron into a molecular orbital that is strongly antibonding with respect to a pair of ligands. In CuCl2(H2O)2 the copper can be described as a highly distorted octahedral complex, the Cu(II) center being surrounded by two water ligands and four chloride ligands, which bridge asymmetrically to other Cu centers.

Properties

Copper(II) chloride dissociates in aqueous solution to give the blue color of [Cu(H2O)6]2+ and yellow or red color of the halide complexes of the formula [CuCl2+x]x-. Concentrated solutions of CuCl2 appear green because of the combination of these various chromophores. The color of the dilute solution depends on temperature, being green around 100 °C and blue at room temperature. When copper(II) chloride is heated in a flame, it emits a green-blue colour.

It is a weak Lewis acid, and a mild oxidizing agent. It has a crystal structure consisting of polymeric chains of flat CuCl4 units with opposite edges shared. It decomposes to CuCl and Cl2 at 1000 °C:

However "soft" ligands such as phosphines (e.g., triphenylphosphine), iodide, and cyanide as well as some tertiary amines cause reduction to give copper(I) complexes. To convert copper(II) chloride to copper(I) derivatives it is generally more convenient to reduce an aqueous solution with the reducing agent sulfur dioxide:

CuCl2 can simply react as a source of Cu2+ in precipitation reactions for making insoluble copper(II) salts, for example copper(II) hydroxide, which can then decompose above 30 °C to give copper(II) oxide:

Uses

A major industrial application for copper(II) chloride is as a co-catalyst (along with palladium(II) chloride) in the Wacker process. In this process, ethene (ethylene) is converted to ethanal (acetaldehyde) using water and air. In the process PdCl2 is reduced to Pd, and the CuCl2 serves to re-oxidise this back to PdCl2. Air can then oxidise the resultant CuCl back to CuCl2, completing the cycle.

This reaction is performed in a polar solvent such as DMF, often in the presence of lithium chloride, which speeds up the reaction rate.

CuCl2, in the presence of oxygen, can also oxidise phenols. The major product can be directed to give either a quinone or a coupled product from oxidative dimerisation. The latter process provides a high-yield synthesis of 1,1-binaphthol (also called BINOL) and its derivatives, these can even be made as a single enantiomer in high enantiomeric excess:

Copper(II) chloride is also used in pyrotechnics as a blue/green coloring agent.

Natural occurrence

Copper(II) chloride occurs naturally as the very rare mineral tolbachite and the dihydrate eriochalcite. Both are known from fumaroles. More common are mixed oxyhydroxide-chlorides like atacamite Cu2(OH)3Cl, arising among Cu ore beds oxidation zones in arid climate (also known from some altered slags).

References

Further reading

Lide, David R. (1990). CRC handbook of chemistry and physics: a ready-reference book of chemical and physical data. Boca Raton: CRC Press.